Fixed wireless access systems are currently employed for local telecommunication networks, such as the IONICA system. Known systems comprise an antenna and decoding means which are located at a subscriber's premises, for instance adjacent a telephone. The antenna receives the signal and provides a further signal by wire to a decoding means. Thus subscribers are connected to a telecommunications network by a radio link in place of the more traditional method of copper cable. Such fixed wireless access systems will be capable of delivering a wide range of access services from POTS (public operator telephone service), ISDN (integrated services digital network) to broadband data. The radio transceivers at the subscribers premises communicate with a base station, which provides cellular coverage over, for example, a 5 km radius in urban environments. A typical base station will support 500-2000 subscribers. Each base station is connected to a standard PSTN switch via a conventional transmission link/network.
When a fixed wireless access telecommunications system is initially deployed, then a base station of a particular capacity will be installed to cover a particular populated area. The capabilities of the base station are designed to be commensurate with the anticipated coverage and capacity requirement. Subscribers' antennas will be mounted outside, for instance, on a chimney, and upon installation will normally be directed towards the nearest (or best signal strength) base station or repeater antenna (any future reference to a base station shall be taken to include a repeater). In order to meet the capacity demand, within an available frequency band allocation, fixed wireless access systems divide a geographic area to be covered into cells. Within each cell is a base station through which the subscriber' stations communicate; the distance between the cells being determined such that co-channel interference is maintained at a tolerable level. When the antenna on the subscriber premises is installed, an optimal direction for the antenna is identified using monitoring equipment. The antenna is then mounted so that it is positioned towards the optimal direction.
There are a number of alternative ways of providing access to the public telephone network, besides fixed wireless access systems. One method is to use copper or optical fibre cable. However, this involves digging up streets to order to lay cables past all the homes in the service area which is expensive, time consuming and causes noise, dirt, damage to trees and pavements and disrupts traffic. After the initial high investment the telephone company can then only start to recoup its investment as new subscribers join the system over a period of time. Another alternative is cellular radio such as GSM. This has the advantage that the telephones are mobile. However, the system operator has to provide continuous coverage along motorways, in shopping malls, and so on. The low-height omni directional antenna used in mobile systems gives little discrimination against multipath interference, and its low height makes it more susceptible to noise. Also, when a mobile moves it suffers constantly varying multipath interference which produces varying audio quality. Mobile cellular networks also require expensive backhaul networks which consist of expensive switches and an expensive master control centre which handle the movement of mobiles from one cell to another.
Radio systems based on mobile standards with fixed directional antennas are sometimes used to provide access to the public telephone network. The directional antenna discriminates against some of the multipath interference. However, the system still suffers from the disadvantages already mentioned. For example, an expensive backhaul network is required and the speech quality is inferior to a copper wire system.
Fixed wireless access systems comprise a basestation serving a radio cell of up to 15 km radius (for example). The basestation interfaces with the subscriber system via a purpose designed air interface protocol. The basestation also interfaces with the public telephone network for example, this interface can be the ITU G.703 2048 kbit/s, 32 timeslot, 30 channel standard known as E1 or the North American 24 timeslot standard known as T1.
Typically, each uplink radio channel (i.e. from a subscriber antenna to a base station) is paired with a downlink radio channel (i.e. from a base station to a subscriber antenna) to produce a duplex radio channel. For voice signals the up and down link channels in a pair normally have the same frequency separation (e.g. 50 MHz between uplink and downlink channels) because this makes the process of channel allocation simple. However, it is possible for the up and down link channels in a pair to have different frequency separations. Often each downlink transmits continuously and it is usual for those downlink bearers used to carry broadcast information to transmit continuously. In the uplink each subscriber antenna typically only transmits a packet of information when necessary.
A bearer is a frequency channel, often with several logical channels, for example, ten channels. Basestations are then allocated radio bearers from the total available, for example, 54. As the subscriber population increases the basestation capacity can be increased by increasing the number of bearers allocated to it, for example, 3, 6 or 18 bearers.
As already mentioned, fixed wireless access systems divide a geographic area to be covered into cells. For initial planning and design purposes these cells are usually represented as hexagons, each cell being served by a base station (in the centre of the hexagon) with which a plurality of subscriber stations within the cell (hexagon) communicate. When detailed cell planning is performed the ideal hexagonal arrangement can start to break down due to site constraints or for radio propagation reasons. The number of subscriber stations which can be supported within each cell is limited by the available number of carrier frequencies and the number of channels per frequency.
Base stations are expensive, and require extensive effort in obtaining planning permission for their erection. In some areas, suitable base station sites may not be available. One problem in fixed wireless access system design is to have as few base stations as possible, whilst supporting as many subscriber stations as possible. This helps to reduce the cost per subscriber in a fixed wireless access system. An on-going problem is to increase the traffic carrying capacity of base stations whilst at the same time keeping interference levels within acceptable bounds. This is referred to as trying to optimise or increase the carrier to interference level ratio. By increasing the traffic capacity the number of lost or blocked calls is reduced and call quality can be improved. (A lost call is a call attempt that fails.)
Cells are typically grouped in clusters as shown in FIG. 1. In this example, a cluster of seven cells is shown and for a 6 bearer system, each cell in the cluster may use a different group of 6 frequencies out of the total available (for example, 54). Within each cluster 7.times.6=42 frequencies are each used once. This leaves 12 channels for in-fill if required. Within the cluster all channels are orthogonal, that is, separated by emitter time and/or frequency, and therefore there will be no co-channel interference within this isolated cluster.
FIG. 2 shows how a larger geographical area can be covered by re-using frequencies. In FIG. 2 each frequency is used twice, once in each cluster. Co-channel interference could occur between cells using the same frequencies and needs to be guarded against through cell planning. When the capacity of a cell or cluster is exhausted one possibility is to sectorize each cell. This involves using directional antennas on the base station rather than omnidirectional antennas. The 360.degree. range around the base station is divided up into a number of sectors and bearers are allocated to each sector. In this way more bearers can be added whilst keeping interference down by only using certain frequencies in certain directions or sectors. For example, up to 12 bearers per cell could be added giving a total of 18 bearers and thereby tripling the capacity of each cell (as shown in FIG. 3). With 18 bearers per cell, the number of cells in a cluster drops to three, as shown in FIG. 3. This is because all 54 frequencies are used in the cluster and will be reused in other clusters.
Known approaches for seeking to increase system capacity include frequency planning which involves carefully planning re-use patterns and creating sector designs in order to reduce the likelihood of interference. However, this method is complex and difficult and there is still the possibility that unwanted multipath reflections may cause excessive interference. Frequency planning is also expensive and time consuming and slows down the rate of deployment. Some of the difficulties with frequency planning include that it relies on having a good terrain base and a good prediction tool.